The production of polyesters and copolyesters of dicarboxylic acids and aliphatic glycols has been carried out commercially for decades. Among the earliest general disclosures relating to the production of polyesters and copolyesters is the disclosure in U.S. Pat. No. 2,465,319, issued Mar. 22, 1949. Since the disclosure in U.S. Pat. No. 2,465,319 many variations have been made in the processes employed to produce such polyesters and copolyesters and many catalysts for use in such processes have been disclosed in the patent and non-patent literature.
U.S. Pat. No. 3,546,179, discloses the use of certain compounds containing both silicon and phosphorus for use as polycondensation catalysts for the production of polyesters and copolyesters.
The use of metal halides and several silicon compounds as catalysts in the polycondensation of dicarboxylic acids and aliphatic glycols is disclosed in U.S. Pat. Nos. 4,143,057, 4,254,241, and Re-30,554. The metal halide catalysts disclosed therein are employed with a solvent present. The use of a solvent is generally undesirable since the solvent may interfere with the polycondensation reaction or may actually need to be removed during the polycondensation reaction. In addition, these metal halides generally tend to be hydrolytically unstable and thus require careful handling during their use. Thus, although the catalysts disclosed in U.S. Pat. Nos. 4,143,057, 4,254,241 and Re-30,554 are desirable catalysts it would be even more desirable to have a catalyst that may be used without the use of or with the use of a minimal amount of a solvent and, further, which has a greater degree of hydrolytic stability. Such a hydrolytically stable catalyst which may be employed without the use of solvent is disclosed in copending application U.S. Ser. No. 336,317, filed Dec. 31, 1981, now U.S. Pat. No. 4,361,694, wherein a alkoxyhalotitanate is employed with an silicon and phosphorus containing compound as the catalyst for the polycondensation reaction.
Polyethylene terephthalates are generally prepared by esterifying a dicarboxylic acid, preferably pure terephthalic acid, or transesterifying the corresponding dimethyl ester of terephthalic acid with from 1.5 to 5, and preferably from 1.5 to 4 moles of a diol, such as ethylene glycol, relative to 1 each mole of the dicarboxylic acid component, in the presence of a transesterification catalyst respectively at between about 150.degree. C. and about 250.degree. C. (reaction step I) and subjecting the reaction products thereof to a polycondensation reaction in the presence of polycondensation catalyst(s) at a temperature preferably between about 200.degree. C. and about 300.degree. C., under reduced pressure, preferably less than about 1 millimeter mercury (Hg) (reaction step II).
It has been found that the selection of the polycondensation catalyst not only has a marked influence on the reaction rate of both transesterification and polycondensation reactions but, also, influences the side reactions, the heat stability, and color of the final polyester, i.e., polyethylene terephthalate products. As a result, there has been an intense interest in the development of transesterification and polycondensation catalysts in an effort to overcome the problems of color, thermal stability and toxicity found in many of such catalysts. For example, see R. E. Wilfong in Poly. Sci. 54, 385 (1961).
Among the more widely used metal-containing polycondensation catalysts are compounds of germanium, titanium, and antimony, employed separately or in combination. U.S. Pat. No. 2,578,660 describes the use of germanium and germanium dioxide as polycondensation catalysts. The use of antimony compounds, in combination with certain phosphorus compounds as stabilizers, is disclosed in U.S. Pat. No. 3,441,540 and in East German Pat. Nos. 30,903 and 45,278.
The use of titanium compounds, titanium tetraisopropylate or titanium tetrabutylate, are described as polycondensation catalysts for the preparation of polyesters in U.S. Pat. Nos. 2,727,881, 2,822,348 and 3,075,952 and British Pat. Nos. 775,316, 777,216, 793,222 and 852,061. In addition East German Pat. No. 45,278 discloses the use of titanium compounds in combination with a phosphorus-containing stabilizer. Unfortunately the phosphorus-containing stabilizer disclosed in East German Pat. No. 45278 does little to change yellow-brown discoloration which may occur in the polycondensation products when a titanium catalyst is employed. In addition when a titanium catalyst is employed to prepare a polyethylene terephthalate it is generally observed that if the reaction time is not kept short or if the catalyst concentration is not maintained at a sufficiently low level, generally, the polyester products fails to achieve the desired degree of polycondensation and that a commercially viable product, in terms of its color, cannot be obtained without the use of a stabilizer for the titanium-containing polycondensation catalyst.
To accommodate the generally poor color characteristics of polyesters formed with metal-containing polycondensation catalysts it is desirable to add a stabilizer to the reaction mixture after the transesterification step and after addition of the polycondensation catalyst. It is believed that such a stabilizer inhibits undesirable side reactions without producing loss of desired catalyst activity. In addition, the stabilizer is expected to increase the thermal stability of the end polyester product and also improve the color characteristics thereof, i.e., whiteness of such products. A general description of inhibitors may be found at H. Ludewig, Polyesterfasern (polyester fibers), 2nd Edition Akademie-Verlag Berlin, 1974, in U.S. Pat. No. 3,028,366 and in German Offenlegungsschriften Nos. 1,644,977 and 1,544,986. Such disclosures have mentioned, generally, as stabilizing compositions such compounds as phosphoric acid, meta-phosphoric acid and phosphorous acid and their phenyl esters such as trinonylphenyl phosphate or triphenyl phosphate or triphenyl phosphite. The use of such triphenyl phosphate or triphenyl phosphite stabilization compositions results in polyester products having poor color characteristics and also results in the formation of phenyl byproducts, i.e., byproducts containing phenol or phenyl derived compounds, which may also be undesirable owing to their bactericidal effect on microorganisms in water treatment systems.
U.S. Pat. No. 3,028,366 discloses a process for polymerizing a glycol ester of terephthalic acid in the presence of a metal-containing catalyst preferably antimony oxide relating to the improvement of producing a polymer substantially free from color by use of phosphoric acid, alkyl phosphates, hydroxyalkyl phosphates, and aryl phosphates in an amount less than 0.2 mol percent, based on the terephthalate content of the polymer. The patent employs antimony as the catalyst in the preferred embodiment. Examples 1 and 2 employ tetraisoproplytitanate as the metal-containing catalyst with example 2 employing tributyl phosphate as a modifier.
In general, it is not possible to obtain sufficiently high molecular weight, light colored polyethylene terephthalates by use of phosphoric acid esters as stabilization compositions, such as with triphenyl-derived stabilization compositions. Such has been recognized by the patent literature; see U.S. Pat. No. 4,115,371, column 2, lines 7 to 23. As a result, the use of triphenyl phosphoric acid derivatives as thermal and color stabilizer compositions have not heretofore been deemed commercially useful in the manufacture of polyethylene terephthalates.
U.S. Pat. No. 4,115,371, issued Sept. 19, 1978, discloses a process for the stabilization of a specific titanium-containing polycondensation catalyst i.e., titanium tetrabutylate. The disclosed process employs only titanium tetrabutylate as the polycondensation catalyst and metaphosphoric acid or its alkali or alkaline earth metal salts. The patentee states that the selection of the catalyst is so "delicately balanced that use of titanium tetraisopropylate gives products which, although they are also of high molecular weight, are, however, slightly yellow colored." Example 3 employs titanium tetraisopropylate and sodium metaphosphate and discloses a yellow shade of 10.3. Although the patentee obtained products having b-values of 5.1, it is obvious that the patentee discloses a stabilizer with which only one specific titanium catalyst may be employed. In contrast, the stabilizer of the instant application may be employed with many different polycondensation catalysts.